This invention relates generally to improvements in fluid delivery systems and, more particularly, to a new and improved disposable syringe cartridge for a syringe pump.
The administration of parenteral fluids to human patients conventionally involves use of a solution administration set. The set typically is a disposable plastic product, and comprises a drip chamber adapted to be connected to a fluid source, a length of tubing extending from the chamber to the patient and a valve mechanism, such as a roller clamp on the tubing.
In recent years, a number of electrical monitoring systems, drop flow controllers and infusion pumps have been developed to accomplish the tasks of sensing and regulating the rate of fluid flow into the human body. One such development has been positive pressure infusion pumps of the syringe type, wherein a syringe having a very precise displacement volume is repeatedly filled and emptied on alternate syringe piston strokes during a combined "fill" and "pump" operational cycle, so that control of the rate at which the syringe is filled and emptied provides an accurate means for precise fluid volume delivery over a prescribed period of time. Such syringe pumps are essentially independent of drop flow inaccuracies introduced by I.V. administration sets and provide an overall solution to accurate and stable fluid volume delivery over long periods of time, at both high and low flow rates.
At the heart of the syringe pump is the syringe itself. Such syringes must be sufficiently rugged and reliable to enable repetitive fill and pump strokes over sustained periods of pump operation without leaking or admitting air or pathogens to the interior of the syringe. Where disposable syringes are involved, the syringe should preferably be of relatively simple economical construction, easily handled for insertion into and removal from the remainder of the pumping apparatus and should be mounted in such a fashion as to facilitate removal of air prior to startup. A prior disposable syringe cartridge designed to meet these and other requirements is shown and described in U.S. Pat. No. 3,993,061, issued Nov. 23, 1976, inventor Stephen H. O'Leary, which patent is assigned to the same assignee as the present application.
Basically, the disposable syringe cartridge disclosed in U.S. Pat. No. 3,993,061 includes a molded plastic cylinder having inlet and outlet nipples and defining an interior chamber adapted to slidably receive a plastic piston and integral piston rod. A rubber sealing cap overlies and encases the plastic piston, and defines a conical piston face. The sealing cap includes a pair of resilient annular ribs defining piston sealing rings, and further includes a limp diaphragm conical sealing boot. The dual, spaced apart sealing rings define two point contact along the longitudinal axis of the syringe to enchance axial alignment and stability of the piston and piston rod as the piston slides within the cylinder of the syringe, whereas the sealing boot at the base of the cylinder prevents the intake of air or pathogens through the bottom of the cylinder during repetitive strokes.
The disposable syringe cartridge of U.S. Pat. No. 3,993,061 itself embodies no valving structure, but includes a pair of intake and output I.V. tubes communicating with inlet and outlet nipples, respectively, of the syringe. The remainder of the pumping apparatus drives the syringe and repetitively and sequentially opens and closes the intake and output I.V. tubes by means of a pair of tube pinchers external to the syringe cartridge, the I.V. tubes alternating their opened and closed states, one tube pincher controlling each I.V. tube.
The inlet and outlet nipples of the syringe cartridge of U.S. Pat. No. 3,993,061 extend parallel to the longitudinal axis of the syringe, on opposite sides of the syringe. The interior surface of the cylinder defines, with the piston, a fluid chamber, and the cylinder surface above the piston is sloped upwardly towards the base of the outlet nipple, so that, when the longitudinal axis of the syringe is vertical, gas bubbles will tend to rise to the highest point of the cylinder and out through the outlet nipple for easy removal.
With the syringe cartridge of U.S. Pat. No. 3,993,061, an integral tab extends from the syringe cylinder and provides an operator handle for mounting and removing the syringe from the overall pumping apparatus. The end of the piston rod remote from the piston head is provided with mounting bosses to engage and be retained by a mounting shoe secured to the leading end of a linear drive shaft adapted to be coupled to the piston rod for driving the syringe through successive fill and pump strokes. A second pair of outwardly extending mounting bosses, parallel to the first set of mounting bosses on the piston rod, are integral with the syringe cylinder and are adapted to engage a pair of fixed guide and retaining slots provided in opposite walls of a syringe receiving compartment defined in the pump housing. The intake and output I.V. tubes from the syringe cartridge pass vertically over a pair of tube pincher blades and are clamped in position by a suitable tubing compartment access door which is appropriately latched.
Although the aforedescribed disposable syringe cartridge has performed generally satisfactorily, it was designed under the assumption that care would be taken by medical personnel to always properly install the cartridge assembly in the syringe pump. Specifically it was assumed that the longitudinal axis of the syringe cylinder would be aligned with the axis of reciprocation of the syringe driving system of the pump. It has been found, however, in practice that the syringe cartridge can be subject to misloading, which can result in misalignment of the syringe cylinder axis relative to the axis of reciprocation of the driving system. As a consequence, a fulcrum effect can be created in which the sealing rings of the piston head are forced away from the cylinder wall and fluid leaks past the piston head. Air or pathogens, or both can then be introduced into the syringe pumping chamber, which is of course undesirable.
It has further been found that the aforedescribed syringe cartridge can be improved in other respects, including the ease with which the cartridge can be sterilized, the sealing between the piston head and the cylinder wall, the design and attachment of the flexible boot, and the connection of the I.V. tubes to the syringe nipples, which can sometimes become dislodged.
Accordingly, it has been recognized that there is a need for an improved syringe cartridge that overcomes the foregoing problems, and yet maintains the desirable characteristics of being relatively simple, economical, reliable, stable and accurate. The present invention clearly fulfills this need .